It is perfectly normal that when people have enough sleep, they wake up. You probably go to bed at a particular time, possibly around midnight, which allows you to get a suitable amount of sleep by 9 in the morning at which time you wake up because you don't need any more sleep.
It is also true that people get used to waking up at certain times and it can become a habit. I personally get up at 7:00 am every day and this is normal for me.
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